Rotation sensing is an important application in inertial navigation, geodesic monitoring, and fundamental research such as tests of fundamental physics. High precision rotation sensors are based on the Sagnac effect, which was discovered over a hundred years ago in an effort to prove the existence of the ether. A review of the Sagnac effect has been published in E. J. Post, “Sagnac effect,” Rev. Mod. Phys. 39, 475 (1967).
There are two ways of implementing the Sagnac effect for precision rotation sensing. Passive Sagnac devices are based on interference between two counter-propagating light fields in a closed loop. The rotation response is detected as a phase shift at the output of the loop. The second implementation provides much better sensitivity and dynamic range and is known as an active Sagnac interferometer. Here, the closed loop is made into a laser cavity that helps transform the difference in phase shift experienced by the two counter-propagating optical fields (laser modes) into a frequency shift.
A popular design of an active Sagnac interferometer is based on a ring He—Ne laser, which can have a triangular or square shape (the ring laser gyroscope). These active laser gyroscopes are currently being used in highly demanding applications such as aircraft and satellite navigation. Their level of sensitivity can reach better than part-per-millions of the Earth's rate (which is about 15 degree an hour). The He—Ne laser gyroscope was developed in the early 1960's. It is a satisfactory instrument for some purposes, yet some important features are not favorable for all applications. The device can be expensive, heavy, bulky, sensitive to electromagnetic interference, and has lifetime limitations. Furthermore, these gyroscopes also suffer from the lock-in effect, which results in low sensitivity at low rotation rates.
The lock-in effect generally occurs at very low rotation rates, which is when the frequencies of the counter-propagating laser modes become almost identical. In this case, crosstalk between the counter-propagating beams can result in injection locking, so that the frequency of each beam becomes locked to one another rather than responding to gradual rotation. Although mitigation techniques have been developed such as mechanical dithering to unlock the frequencies, lock-in is still a major issue in laser-based rotation sensors that cannot be dithered.
The frequency shift detected at the output of the active laser gyroscope is proportional to the area covered by the laser cavity. For that reason, it is desirable to have as large a sensing area as possible to achieve greater sensitivity. Experiments on area scaling have been done in the last two decades by various groups, mainly in Europe. One example is the Gross Ring Laser Gyroscope that has about a 16 m2 sensing area. The entire ring laser was made out of a monolithic Zerodur block to improve mechanical stability. As a result, the device can detect rotation of the earth with ultrahigh level of precision such that it has detected the earth tide, tilt, and the subtle wobbling of the earth's rotation axis over time. The biggest active laser gyroscope was developed in 2009 and featured a 39.7×21 m2 (or 834 m2) cavity. However, this is basically the limit of He—Ne laser technology. The large cavity is not stable enough for long term use due to the massive weight of the support structure where the cavity mirrors are mounted. Gravity pulls the heavy cavity so much that the cavity length is drifting over time and cannot be compensated.